The present invention pertains to continouous reactor processes and in particular to the use of continuous stirred tank reactors and/or bubble columns to effect such processes.
Hydrogenation processes wherein three distinct phases, i.e. a material to be hydrogenated, hydrogen gas, and a catalyst are brought together in either a continuous stirred tank reactor (CSTR) or a bubble column are well known in the industry.
In view of the fact that most hydrogenation processes take place at elevated pressure, bubble column reactors are generally less costly than continuous stirred tank reactors. Bubble columns can be constructed taller and narrower than CSTR reactors and therefore require thinner reactor walls, thus lessening the amount of material of construction and the cost. Also, a bubble column requires no mechanical agitation, thus further reducing initial cost and further reducing maintenance costs for such reactors. In a bubble column, hydrogen gas is introduced into the bottom of the column and must be dispersed to create a high gas-liquid contact area for mass transfer and to provide liquid mixing in the column to effect proper reaction and heat transfer. To effect mixing by controlling the flow rates of hydrogen gas, a much greater quantity of hydrogen gas is used than is dictated by stoichiometric conditions for the particular reaction. Thus, excess hydrogen must be used in the process or a hydrogen recovery and recycle apparatus, usually employing a compressor to recycle the hydrogen exiting the column, is used.
Depending upon the throughput of a continuous process, very often a single reactor is not sufficient to provide adequate residence time to generate high conversion in a single reactor and thus two or more reactors in series are required. Carrying out gas-liquid reactions in two or more columns is quite common, however, where three phase slurry hydrogenation is desired using a solid catalyst, use of single reactors becomes a challenge with problems that are not easily overcome. In general hydrogenation catalysts have sufficient activity and life and can remain in a process for days, compared to the liquid residence times of a process which might require only minutes or hours. This large difference in residence times means that the catalyst must be maintained in a uniform distribution when multiple reactors are used. Each reactor may require catalyst separation equipment and the ability to recycle the catalyst.
Thus, the use of a single continuous stirred tank reactor or a bubble column may not have sufficient residence time to maintain high conversion requiring two reactors of the same type in series.
The slurry catalyst in continuous stirred tank reactors must be separated and returned to each individual reactor. Thus multiple reactors in series each require individual catalyst separation equipment such as cross flow filters.
Multiple bubble columns in series can be used to increase overall conversion but require hydrogen flow rates greatly in excess of the stoichiometric requirement to achieve both sufficient mixing and high conversion in the reactor.
Also it is well known that a single continuous stirred tank reactor must be significantly larger and generally more expensive than two smaller continuous stirred tank reactors in series to achieve the same overall conversion.
Thus it can be seen that a multi stage slurry hydrogenation process using conventional technology is problematic and economically disadvantageous, thus creating a need to solve these problems.